1. Field of the Invention
This invention relates to a radiation image detector adapted for use in a radiation imaging apparatus, such as an X-ray imaging apparatus.
2. Description of the Related Art
With respect to radiation imaging operations for medical diagnoses, there have heretofore been known radiation imaging apparatuses, in which radiation image detectors (utilizing semiconductors as principal sections) are utilized for detecting radiation and converting the radiation into an electric signal. Ordinarily, the radiation image detectors may be classified into a direct conversion type, wherein the radiation is directly converted into electric charges, and wherein the thus formed electric charges are accumulated; and an indirect conversion type, wherein the radiation is converted into light by use of a scintillator, such as CsI:Tl or GOS (Gd2O2S:Tb), wherein the thus obtained light is then converted into electric charges by use of a photoconductive layer, and wherein the thus formed electric charges are accumulated. Also, in accordance with read-out techniques, the radiation image detectors may be classified into an optical read-out technique, wherein the electric charges having been generated with the irradiation of the radiation are accumulated at a charge accumulating section, and wherein the accumulated electric charges are read out by the utilization of a semiconductor material capable of generating the electric charges when being exposed to light; and a TFT technique, wherein the electric charges having been generated with the irradiation of the radiation are accumulated at accumulating capacitors, and wherein the accumulated electric charges are read out through an operation, in which an electric switch, such as a thin film transistor (TFT), is turned on and off with respect to each of pixels.
The direct conversion types of the radiation image detectors are constituted for performing a radiation detecting operation, wherein a predetermined bias voltage is applied across a bias electrode, which has been formed on a surface of a radiation-sensitive semiconductor film (acting as a recording photoconductive layer), and a reference electrode, which has been formed on a base plate, wherein the electric charges having been generated with the irradiation of the radiation are collected by a charge collecting electrode, which been formed on an opposite surface of the semiconductor film, and wherein the collected electric charges are taken out as a radiation detection signal. Ordinarily, the recording photoconductive layer is formed with amorphous selenium (a-Se) for its advantages of a high dark resistance and a high response speed.
Ordinarily, from the view point of environmental stability, as the material of the electrode described above, a metal material having a large work function (of, approximately, 5 eV), such as Pt or Pd, is employed. The work function of a-Se is equal to approximately 5.8 eV, and the difference between the work function of a-Se and the work function of the material of the electrode described above is thus small. Therefore, in cases where the electrode is formed on a-Se, and a bias voltage for yielding a positive potential is applied, a dark current arises due to holes having been injected from the electrode to a-Se by being assisted by the electric field. After the holes have once been injected, the surplus holes contribute to a large dark current, which markedly exceeds a high specific resistance of a-Se.
Therefore, ordinarily, radiation image detectors are provided with a hole capture layer. For example, in U.S. Pat. Nos. 5,880,472 and 6,171,643, there is described a technique, wherein an a-Se layer having been doped with Na (which is one of alkali metals) is provided as a hole capture layer (having a thickness falling within the range of 0.5 μm to 10 μm) for capturing the holes, which have been injected from an electrode on the positive bias application side into a photoelectric conversion layer, and for reducing a dark current.
Also, for example, in Japanese Patent No. 3774492, there is described a technique, wherein an a-Se layer having been doped with CaF2 (concentration: falling within the range of 0.05% to 0.5%, film thickness: falling within the range of 0.05 μm to 1 μm), LiF2 (concentration: falling within the range of 0.05% to 10%, film thickness: falling within the range of 0.05 μm to 1 μm), or LiF (concentration: failing within the range of 0.05% to 0.5%, film thickness: falling within the range of 0.05 μm to 1 μm) is provided as a hole capture layer for capturing the holes, which have been injected from an electrode on the positive bias application side into a photoelectric conversion layer, and for reducing a dark current. Further, it is described that, as the dopants to be used in the hole capture layer, besides CaF2 (which is one of alkaline earth metal fluorides), LiF, and LiF2 (which is one of alkali metal fluorides) described above, metal elements, such as Li, Na, K, Mg, Ca, Ba, and Tl, are equally efficient.
With the hole capture layer described in U.S. Pat. Nos. 5,880,472 and 6,171,643, though the initial electric characteristics (the dark current characteristics) are capable of being satisfied, it is not always possible to keep the electric characteristics (the dark current characteristics and the defect characteristics) over a long period of time. It is presumably since the alkali metal is apt to diffuse in a-Se, the distribution of the doping concentration alters with the passage of time, the alkali metal acting as the doping substance migrates to an a-Se interface, and crystallization is thus apt to occur.
With the hole capture layer described in Japanese Patent No. 3774492, the initial electric characteristics (the dark current characteristics: at a stage during voltage application or at a stage immediately after voltage short-circuiting) are capable of being satisfied. However, in cases where the compound depart, such as CaF2, LiF, or LiF2, which is employed in Japanese Patent No. 3774492, is doped in a molar concentration falling within the range of 0.05% to 10%, it is not always possible to keep the quality (the defect characteristics) over a long period of time.
It has been found that the problems described above are encountered for the reasons described below. Specifically, with the doping technique, wherein an alloy raw material, which is constituted of a mixture of an Se raw material and the compound dopant, is utilized as the deposition raw material, since a melting temperature of the compound dopant described above is ordinarily as high as at least 800° C., the efficiency with which the compound dopant is doped into a vacuum deposited film is not capable of being kept high. Therefore, it is necessary to utilize a co-vacuum evaporation technique, wherein the compound dopant described above is doped by use of the vacuum evaporation from a hearth different from the hearth for the Se raw material. With the co-vacuum evaporation technique, the defects, such as crystallization nucleuses, are apt to be introduced into the a-Se film acting as the matrix of the hole capture layer.